A lost toponym is a name given to a place which is no longer known or identifiable.
[1] For example, in 1618 the crew of the Mauritius explored a river in north-western Australia, which they named Willems Revier ("Willem's River").
However it is not now known which river they explored, so Willems River cannot be placed on a present-day map.
Willems River is therefore a lost toponym.
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